Mechanical waves are waves that require a medium. This means that they have to have some sort of matter to travel through. These waves travel when molecules in the medium collide with each other passing on energy. One example of a mechanical wave is sound. Sound can travel through air, water, or solids, but it can't travel through a vacuum. It needs the medium to help it travel. Other examples include water waves, seismic waves, and waves traveling through a spring.
Hi Kelly, I'm now 28 weeks pregnant with twins and I first started feeling them move when I was around 15/16 weeks pregnant but this is my second pregnancy. In my first pregnancy (not twins) I didn't feel any movement until I was 24 weeks pregnant. I think everyone feels their babies at different times so don't worry about anything.
This is the start of the fifth month in the second trimester of pregnancy. Week 21: The baby will be moving a lot now and you may also feel when your fetus has hiccups. They are rhythmical tapping sensations, different to kicks. The heartbeat will also be louder, beating at about 120 to 140 per minute.